Advanced Java Programming After mastering the basics of

Advanced Java Programming After mastering the basics of Java you will now learn more complex but important programming concepts as implemented in Java.

References: What You Should Know • In Java when you use objects and arrays there are two things involved: – Reference – Object (or array) • Example with an object Person charlie; // Creates reference to object charlie = new Person(“Sheen”); // Creates object • Example with an array double [] salaries; // Creates reference to array salaries = new double[100]; // Creates array James Tam

Reminder: Variables • Think of the ‘mail box’ metaphor • A slot containing a reference (and not data) will contain the number of another ‘slot’ (address) in memory. 1002 Pictures from Computers in your future by Pfaffenberger B James Tam

References (Can) Contain Addresses Person jim; jim* RAM Address 200, 000 Null address 100, 000 100, 001 … (Person object) 200, 000 jim = new Person(); James Tam

Shallow Copy Vs. Deep Copies • Shallow copy (new term, concept should be review) A shortcut (‘link’ in UNIX) is similar to a shallow copy. Multiple things that refer to the same item (document) – Copy the address from one reference into another reference – Both references point to the same location in memory James Tam

Shallow Copy Vs. Deep Copies (2) • Shallow copy, full example under: /home/233/examples/advanced/1 shallow. Deep Person mary = new Person(21); Person bob = new Person(12); System. out. println(mary. age + " " + bob. age); mary = bob; // Shallow; bob. age = 66; System. out. println(mary. age + " " + bob. age); mary Age 21 bob 66 Age 12 James Tam

Shallow Copy Vs. Deep Copies (3) • Deep copy (new term, concept should be review) Making an actual physical copy is similar to a deep copy. – Don’t copy addresses stored in the references – Instead the data referred to by the references are copied – After the copy each reference still refers to a different address (data variable) James Tam

Shallow Copy Vs. Deep Copies (4) • Deep copy, full example under: /home/233/examples/advanced/1 shallow. Deep // Mary still 66 bob = new Person(77); mary. age = bob. age; // Deep bob. age = 144; System. out. println(mary. age + " " + bob. age); mary bob 66 Age 77 77 Age 144 James Tam

Methods Of Parameter Passing • Pass by value – The data stored (the “value” stored) in the parameter is copied • Pass by reference – Pass the address of the parameter – This allows references to the parameter inside the method (the method has a “reference” to the original parameter). James Tam

Passing Parameters As Value Parameters method (p 1); Pass a copy of the data method (<parameter type> <p 1>) { } James Tam

Passing Parameters As Reference Parameters method (p 1); Pass the address of the parameter (refer to the parameter in the method) method (<parameter type> <p 1>) { } James Tam

Which Parameter Passing Mechanism Is Used? Passed by value Pass by reference • All ‘simple’ built in types: • Objects • Arrays • (That is anything that consists of a reference and the item referenced). – Integers (byte, short, int, long) – Floating point (float, double) – Character (char) – Boolean (boolean)

Parameter Passing Example • Full example under: /home/233/examples/advanced/2 parameters James Tam

Class Person public class Person { private int age; private String name; public Person() { age = -1; name = "none"; } public int get. Age() { return(age); } public String get. Name() { return(name); } James Tam

Class Person (2) public void set. Age(int an. Age) { age = an. Age; } public void set. Name(String a. Name) { name = a. Name; } } James Tam

Class Parameter. Example public class Parameter. Example { public void modify(Person a. Person, int a. Num) Modifies { parameters here a. Person. set. Name("Eric Cartman"); a. Person. set. Age(10); a. Num = 888; System. out. println("Person inside modify()"); System. out. println(a. Person. get. Name() + " " + a. Person. get. Age()); System. out. println("Number inside modify()"); System. out. println(a. Num); } } James Tam

The Driver Class public class Driver { public static void main(String [] args) { int num = 13; Person a. Person = new Person(); Parameter. Example pe = new Parameter. Example(); System. out. println("Person in main() before edit"); System. out. println(a. Person. get. Name() + " " + a. Person. get. Age()); System. out. println("Number inside main() before edit"); System. out. println(num); System. out. println("-----"); James Tam

The Driver Class (2) pe. modify(a. Person, num); System. out. println("-----"); System. out. println("Person in main() after edit"); System. out. println(a. Person. get. Name() + " " + a. Person. get. Age()); System. out. println("Number inside main() after edit"); System. out. println(num); } } James Tam

Previous Example: Analysis • Why did the parameter that was passed by reference change and the simple type (passed by value) did not? James Tam

Benefits Of Employing References • References require a bit more complexity but provide several benefits over directly working with objects and arrays. • Benefit 1: As you have just seen a reference contains the address of ‘something’ (object, array). – As long as the address of the object or array is retained changes made inside the method will persist. – Recall that functions or methods can only return zero or one things (passing out of a function after it ends). – Passing by reference (passing into the function just as it starts executing) allows more than one change to persist after the function has ended: fun (refernce 1, reference 2, reference 3…etc. ) James Tam

Benefits Of Employing References (2) • Benefit 2: If an array or object is large then it may be much more memory efficient to pass a reference instead. • Example: – References are typically 32 or 64 bits in size. – An array or object will almost always be larger. char [] array 1 = new char[1000000]; // 2 MB class Social. Network. User { // attribute for images // attribute for videos } James Tam

Modifying Simple Types (Parameters) • Only one thing to be changed: return the updated value after the method ends) • More than one thing to be changed: – Pass an array (e. g. , three integers must be modified in a method then pass an array of integers with 3 elements). – Employ a wrapper (class). James Tam

Wrapper Class • A class definition built around a simple type e. g. , public class Integer. Wrapper { private int num; public int get. Num () { return num; } public void set. Num (int new. Num) { num = new. Num; } } • Also Wrapper classes are also used to provide class-like capabilities (i. e. , methods) to simple variable types e. g. , class Integer –http: //docs. oracle. com/javase/6/docs/api/java/lang/Integer. html –Example useful method parse. Int(String): converting strings to integers int num = Integer. parse. Int(“ 123”); // More on this later James Tam

Arrays: Parameters And Return Values • Full example under: /home/233/examples/advanced/3 array. Parameters • Format, method call: –When the method is called, passing an array as a parameter and storing a return value appears no different as other types. –Example (list 1 and list 2 are arrays) list 2 = ape. one. Dimensional(list 1); James Tam

Arrays: Parameters And Return Values (2) • Format, method definition: – Use ‘square brackets’ to indicate that the return value or parameter is an array. – Each dimension requires an additional square bracket. – One dimensional: public int [] one. Dimensional(int [] array 1) – Two dimensional: public char [][] two. Dimensional(char [][] array 1) James Tam

Array Of ‘Objects’ • Although referred to as an array of objects they are actually arrays of references to objects. • Recall for arrays 2 steps are involved to create array int [] array; array = new int[3]; // Reference to array // Creates array of integers • Recall for objects 2 steps are also required to create object Person jim; jim = new Person(); // Reference to Person object // Creates object James Tam

Array Of ‘Objects’ (2) • An array of objects is actually an array of references to objects. • So 3 steps are usually required – Two steps are still needed to create the array // Step 1: create reference to array Person [] some. People; // Step 2: create array some. People = new Person[3]; • In Java after these two steps each array element be null. some. People[0]. set. Age(10); // Null pointer exception – The third step requires traversal through array elements (as needed): create a new object and have the array element refer to that object. • (The third step can typically be skipped for array elements that are supposed to be ‘empty’) James Tam

Array Of ‘Objects’ (3) – (Step 3: creating objects continued) for (i = 0; i < 3; i++) { // Create object, array element refers to that object some. People[i] = new Person(); // Now that array element refers to an object, a method // can be called. some. People[i]. set. Age(i); } James Tam

Array Of Objects: Example • Location of the full example: – /home/233/examples/advanced/4 array. References/simple James Tam

Class Person public class Person { private int age; public Person() { age = 0; } public int get. Age() { return(age); } public void set. Age(int an. Age) { age = an. Age; } } James Tam

Driver Class public class Driver { public static void main(String [] args) { Person [] some. People; // Reference to array int i; some. People = new Person[3]; // Create array for (i = 0; i < 3; i++) { // Create object, each element refers to a newly // created object some. People[i] = new Person(); some. People[i]. set. Age(i); System. out. println("Age: " + some. People[i]. get. Age()); } } James Tam

Design Example • Suppose we wanted to simulate a 2 D universe in the form of a numbered grid (‘World’) class World { private [][] Tardis; } • Each cell in the grid was either an empty void or contained the object that traveled the grid (‘Tardis’)1 class Tardis { } 1 Tardis and “Doctor Who” © BBC James Tam

General Description Of Program • • The ‘world/universe’ is largely empty. Only one cell contains the Tardis. The Tardis can randomly move from cell to cell in the grid. Each movement of Tardis uses up one unit of energy James Tam

Designing The World Class Tardis • Attributes? • Methods?

CAUTION: STOP READING AHEAD • JT’s note: Normally you are supposed to read ahead so you are prepared for class. • In this case you will get more out of the design exercise if you don’t read ahead and see the answer beforehand. • That will force you to actually think about the problem yourself (and hopefully get a better feel for some design issues). • So for now skip reading the slides that follow this one up to the one that has a corresponding ‘go’ symbol all over it. • After we have completed the design exercise in class you should go back and look through those slides (and the source code). James Tam

Tardis • Attributes – Current energy level • Methods: – Randomly generating movement: • Some method must reduce the energy level as the Tardis moves • The actual ‘movement’ from square to square in the grid will be a responsibility of class World because the grid is an attribute of the world. James Tam

World • Attributes – – – A 2 D array that stores information about the ‘universe’ Most array elements will be empty (null) One element will refer to the Tardis object The maximum number of rows and columns The current location (row/column ) of the Tardis • Needed to ‘move’ the Tardis from source cell to destination cell [0] [1] [0] Null [1] Null Tardis object – Theoretically the (row/col) could be (int, int) but because at most one item can be returned from a method the location will be tracked as 1 D integer array (details in code): • World. move()->Tardis. calculate. Coordinates() James Tam

World (2) • Methods – Constructor(s) to create the world – Methods that modify the world (e. g. , making sure each array element is truly null: wipe() – Displaying the world: display() – Changing the contents of the objects in the world (e. g. , editing the world or moving objects): move() James Tam

Manager • It is responsible for things like determining how long the simulation runs. • For very simple programs it may be a part of the World class (in this case it’s part of the Driver). • But more complex programs (e. g. , need to track many pieces of information like multiple players, current scores etc. and simulation rules) may require a separate Manager class. – The Driver will then likely be responsible for instantiating a Manager object and calling some method of the manager to start the simulation. James Tam

END SECTION: Proceed Reading • You can continue reading ahead to the slides that follow this one. – JT: Thank you for your understanding and co-operation. James Tam

Source Code: Design Exercise • Location of the full source code: /home/233/examples/advanced/4 array. References/doctor James Tam

Class Tardis public class Tardis { private int energy; 0 1 2 3 4 5 6 0 1 2 3 public Tardis(int start. Energy) { energy = start. Energy; } // max row and column define the size of the world public int[] calculate. Coordinates(int max. Row, int max. Column) { Random a. Generator = new Random(); e. g. , = 4 e. g. , = 6 int [] new. Coordinates = new int[2]; 0, 1, 2, 3 new. Coordinates[0] = a. Generator. next. Int(max. Row); new. Coordinates[1] = a. Generator. next. Int(max. Column); 0, 1, 2, 3, 4, 5 energy--; return(new. Coordinates); } } James Tam

Class World: Attributes public class World { private Tardis [][] grid; // Simulated world private int max. Row; // Row capacity private int max. Column; // Column capacity private int [] current. Location; // (row/col) of Tardis James Tam

Class World: Constructor public World() { // Element 0: current row the tardis is located // Element 1: current column the tardis is located current. Location = new int[2]; Scanner in = new Scanner(System. in); System. out. print("Max rows: "); max. Row = in. next. Int(); System. out. print("Max columns: "); max. Column = in. next. Int(); grid = new Tardis[max. Row][max. Column]; wipe(); // Empties the world, sets everything to null grid[0][0] = new Tardis(10); // Tardis starts top left current. Location[0] = 0; // Tardis row = 0 current. Location[1] = 0; // Tardis col = 0 display(); } James Tam

Class World: Initialization public void wipe() { int r; int c; e. g. , max = 2 for (r = 0; r < max. Row; r++) { e. g. , max = 3 for (c = 0; c < max. Column; c++) { grid[r][c] = null; [0] } } null r = 0, c = {0, 1, 2} [0] } null r = 1, c = {0, 1, 2} [1] [2] null James Tam

Class World: Display 0 1 2 3 4 5 6 0 public void display() 1 { 2 int r; 3 int c; e. g. , = 4 for (r = 0; r < max. Row; r++) { e. g. , = 6 for (c = 0; c < max. Column; c++) { if (grid[r][c] == null) System. out. print(". "); else System. out. print("T"); } System. out. println(); Move cursor to display new row on next line } } James Tam

Movement • To make it look like the Tardis has ‘moved’. • Set the destination (row/column) to refer to the Tardis object. • Set the source (row/column) to null Before move [0] [1] [0] Null [1] Null After move Tardis object [0] [1] [0] Null [1] Null Tardis object James Tam

Class World: Move public void move() { // current. Location 1 D array stores Tardis location int current. Row = current. Location[0]; int current. Column = current. Location[1]; // Keep track of where the Tardis is currently located int old. Row = current. Row; int old. Column = current. Column; // Store new (row/col) in 1 D array (current. Location) current. Location = grid[current. Row][current. Column]. calculate. Coordinates (max. Row, max. Column); Recall: Tardis. current. Coordinates() randomly generates a new (row/column) location James Tam

Class World: Move (2) // Update temporary values with current location current. Row = current. Location[0]; current. Column = current. Location[1]; // Copy tardis from the old location to the new one. grid[current. Row][current. Column] = grid[old. Row][old. Column]; // // // if Check if tardis trying to move onto same square, don't 'wipe' if this is the case or tardis will be lost (Tardis object becomes a memory leak). ((current. Row == old. Row) && (current. Column == old. Column)) { System. out. println(“Same location"); } else { // ‘wipe’ tardis off old location grid[old. Row][old. Column] = null; } James Tam

Class World: Move (3) System. out. println("Tardis re-materializing"); display(); } James Tam

The Driver Class (Also The “Manager”) public class Driver { public static void main(String [] args) { Scanner in = new Scanner(System. in); World a. World = new World(); int i; for (i = 0; i < 10; i++) { a. World. move(); System. out. println("Hit enter to continue"); in. next. Line(); } System. out. println("n<<<Tardis is out of energy, end simulation>>> n"); } } James Tam

Universally Accessible Constants • What you currently know – How to declare constants that are local to a method class Driver { main() { final int A_CONST = 10; } } • If you need constants that are accessible throughout your program then declare them as class constants. James Tam

Declaring Class Constants • Format: public class <class name> { public final <type> <NAME> = <value>; } • Example: public class Person { public final int MAX_AGE = 144; } • Note: Because constants cannot change it is okay to set the access level to public. James Tam

Accessing Class Constants • Format (outside of the class definition)1: <class name>. <constant name>; • Example (outside of the class definition): main() { System. out. println(“Max life span: “ + Person. MAX_AGE); } • Accessing a constant inside the methods of that class do not require the name of the class public class Person { … public void fun() { System. out. println(MAX_AGE); } } James Tam

Introducing A New Concept With. . Class Sheep! public class Sheep { private String name; public Sheep() { name = "No name"; } public Sheep(String a. Name) { set. Name(a. Name); } public String get. Name() { return name; } public void set. Name(String new. Name) { name = new. Name; } } James Tam

We Create Several Sheep I’m Jim! I’m Bill! I’m Nellie! James Tam

Question: Who Tracks The Size Of The Herd? Jim: Me! Bill: Me! Nellie: Me! James Tam

Answer: None Of The Above! • Information about all instances of a class should not be tracked by an individual object. • So far we have used instance fields. • Each instance of an object contains it’s own set of instance fields which can contain information unique to the instance. public class Sheep { private String name; . . . } Object name: Bill Object name: Jim Object name: Nellie James Tam

The Need For Static (Class Attributes) • Static fields: One instance of the attribute exists for the class (not one attribute for each instance of the class) Class Sheep flock. Size Object name: Bill Object name: Jim Object name: Nellie James Tam

Static (Class) Methods • Are associated with the class as a whole and not individual instances of the class. –Can be called without having an instances (because it’s called through the class name not a reference/instance name). –Instance method: Scanner in = new Scanner(System. in); in. next. Int(); // ref. Name. method() –Class Method: double square. Root = Math. sqrt(9); // Class. Name. method() • Typically implemented for classes that are never instantiated e. g. , class Math. James Tam

Accessing Static Methods/Attributes • Inside the class definition Format: – <attribute or method name> Example: public Sheep () { flock. Size++; } James Tam

Accessing Static Methods/Attributes (2) • Outside the class definition Format: <Class name>. <attribute or method name> Example: Sheep. get. Flock. Size(); James Tam

Static Data And Methods: UML Diagram • Location of the online example: –/home/233/examples/advanced/5 class. Attributes Static attribute is specified using underlining Driver Sheep -flock. Size: int -name: String +Sheep() +Sheep(a. Name: String) +get. Flock. Size(): int +get. Name(): String +set. Name(a. Name: String): void James Tam

Static Data And Methods: The Driver Class public class Driver { public static void main(String [] args) { System. out. println(); System. out. println("You start out with " + Sheep. get. Flock. Size() + " sheep"); System. out. println("Creating flock. . . "); Sheep nellie = new Sheep("Nellie"); Sheep bill = new Sheep("Bill"); Sheep jim = new Sheep(); System. out. println("Current count " + Sheep. get. Flock. Size()); } } James Tam

Static Data And Methods: The Sheep Class public class Sheep { private static int flock. Size = 0; private String name; public Sheep() { flock. Size++; name = "No name"; } public Sheep(String a. Name) { flock. Size++; set. Name(a. Name); } public static int get. Flock. Size () { return flock. Size; } public String get. Name() { return name; } public void set. Name(String new. Name) { name = new. Name; } } James Tam

Rules Of Thumb: Instance Vs. Class Fields • If a attribute can differ between instances of a class: –The field probably should be an instance field (non-static) • If the attribute field relates to the class (rather to a particular instance) or to all instances of the class –The field probably should be a static field of the class James Tam

Rule Of Thumb: Instance Vs. Class Methods • If a method can be invoked regardless of the number of instances that exist (e. g. . , the method can be run when there are no instances) then it probably should be a static method. • If it never makes sense to instantiate an instance of a class then the method should probably be a static method. – E. g. , the class doesn’t have any variable attributes only static constants such as class Math • Otherwise the method should likely be an instance method. James Tam

Static Vs. Final • Static: Means there’s one instance of the attribute for the class (not individual instances for each instance (object) of the class) • Final: Means that the attribute cannot change (it is a constant) public class Foo { public static final int num 1= 1; private static int num 2; /* Rare */ public final int num 3 = 1; /* Why bother (waste) */ private int num 4; : : } James Tam

An Example Class With A Static Implementation public class Math { // Public constants public static final double E = 2. 71… public static final double PI = 3. 14… // Public methods public static int abs (int a); public static long abs (long a); : : } • For more information about this class go to: – http: //docs. oracle. com/javase/7/docs/api/java/lang/Math. html James Tam

Should A Class Be Entirely Static? • Generally it should be avoided if possible because it often bypasses many of the benefits of the Object-Oriented approach. • Usually purely static classes (cannot be instantiated) have only methods and no data (maybe some constants). • Example (purely for illustration): Math math 1 = new Math(); Math math 2 = new Math(); // What’s the difference? Why bother? math 1. abs() vs. math 2. abs(); • When in doubt DO NOT make attributes and methods static. James Tam

What You Should Know: Attributes Vs. Locals • Attributes: Defined inside a class definition but outside the body of a method. class Person { private int age; } • Locals: Defined inside the body of a method. class Person { public void Person(){ Scanner in = new Scanner(System. in); } } James Tam

Reminder: Scope • Attributes – Declared within the body of a class but outside a method – Accessible anywhere with the class methods class Person { private int age; public Person() { age = 12; }. . . } Scope of attributes and methods • Local variables – Declared inside the body of a method and only accessible in that method class Person { public Person () { Scanner in = new Scanner(System. in); } Scope of locals James Tam

Scoping Rules • When there is a reference to an identifier (e. g. , variable or constant) apply the following scoping rules: – First look for a local variable – Then look for an attribute • Example: class Person { SECOND: • Look for an attribute with this name • i. e. , private int age; public set. Age() { FIRST: • Look for a local variable with this name • i. e. , int age; age = 123; } } Reference to an identifier James Tam

New Terminology: Shadowing • There is an attribute and local variable with the same name. • The local variable ‘shadows’ or hides access to the attribute (unless the ‘this’ reference is used – coming up later). class Person { private int age; public Person() { int age = 123; } } // Common logic error! James Tam

Self Reference: The ‘This’ Reference • From every (non-static) method of an object there exists a reference to the object (called the “this” reference) 1 main(String args []) { Person fred = new Person(); Person barney = new Person(); fred. set. Age(35); } public class Person { private int age; public void set. Age(int an. Age) { age = an. Age; } … } This is one reason why methods must be invoked via a reference name (the contents of the reference ‘fred’ will be copied into the ‘this’ reference (so both point to the ‘Fred’ object). The ‘this’ reference is implicitly passed as a parameter to all nonstatic methods. One use of ‘this’ is to distinguish which object’s method is being invoked (in this case Fred vs. Barney) 1 Similar to the ‘self’ keyword of Python except that ‘this’ is a syntactically enforced name. James Tam

The ‘This’ Reference Is Automatically Referenced Inside (Non-Static) Methods public class Person { private int age; public void set. Age(int an. Age) { // These two statements are equivalent age = an. Age; this. age = an. Age; } } James Tam

New Terminology • Explicit parameter(s): explicitly passed (you can see them when the method is called and defined). fred. set. Age(10); // 10 explicit barney. set. Age(num); // num explicit public void set. Age(int age) {. . . } // age explicit • Implicit parameter: implicitly passed into a method (automatically passed and cannot be explicitly passed): the ‘this’ reference. public void set. Age(int age) {. . . } // ‘this’ is implicit James Tam

Benefits Of ‘This’: Attributes • Another side benefit is the this reference can make it very clear which attributes are being accessed/modified. public class Person { private int age; Parameter (local variable) ‘age’ public void set. Age(int age) { this. age = age; } } Attribute ‘age’ James Tam

Benefits Of ‘This’: Parameters • Another side benefit is the this reference can make it clear which object is being accessed e. g. , when a class method takes as a explicit parameter an instance of that class 1 main (String [] args) { Person fred = new Person(); Person barney = new Person(); barney. name. Best. Buddy(fred); // JT: Explicit? Implicit? } // JT: What will be the output? public void name. Best. Buddy(Person a. Person) { println(this. name + “ best friend is “ + a. Person. name); } 1 JT: more on this one later – see the ‘equals()’ method James Tam

Benefits Of ‘This’: Scope • Recall: according to scoping rules, local variables are not accessible outside of that function or method (unless returned back to the caller or passed into another method). main (String [] args) { int age = 27; Person jim = new Person(); jim. set. Age(age); } class Person { public void set. Age(int age) { this. age = age; } Normally the object referred to by the ‘jim’ reference not accessible outside of main() but the ‘this’ reference contains it’s address (implicit pass by reference) main() age 27 jim (imp) . age 27 (exp) jim. set. Age( this ) age 27 James Tam

Static Methods: No ‘This’ Reference • Recall: static methods do not require an object to be instantiated because they are invoked via the class name not a reference name. int result = Math. abs(-12); • That means static methods do not have the implicit ‘this’ parameter passed in. • Also recall I said for now avoid [for the ‘Driver’ class]: – Defining attributes for the Driver – Defining methods for the Driver (other than the main method) James Tam

Static Methods: No ‘This’ Reference (2) public class Driver { private int num; public static void main(String [] args) { num = 123; } } Driver. java: 6: error: non-static variable num cannot be referenced from a static context • Main() must be static! Automatically called when the program runs via ‘java Driver’ before any other code. • If main() were non-static it would require an object to be instantiated (which must occur inside of a method). • But there would be no way to call that method that instantiates an object without a starting static method. • Because main() must be static, it has no ‘this’ implicit parameter which in turn means that non-static attributes like ‘num’ cannot be accessed (although static attributes/methods accessible): Driver. static_name or just via static_name James Tam

Mutable Vs. Immutable Types • Mutable types – Original memory can be modified int num = 666; num = 777; • Immutable types – The original memory location cannot be modified – Assigning new values will create a new memory location and leave the original untouched. String s 1 = “abc”; String s 2 = s 1; s 1 = “xyz”; System. out. println(s 1 + “ “ + s 2); James Tam

Mutable Vs. Immutable • Advantage of mutable types: speed • Advantage of immutable types: ‘security’ James Tam

Mutable Advantage: Speed • Location of full examples: – /home/233/examples/advanced/6 mutable. Immutable/speed public class String. Example { public static void main (String [] args) { String s = "0"; int i; for (i = 1; i < 100000; i++) s = s + i; } } public class String. Buffer. Example { public static void main (String [] args) { String. Buffer s; int i; s = new String. Buffer("0"); for (i = 1; i < 100000; i++) s = s. append(i); } } James Tam

Immutable Advantage: Security • Location of the full example: – /home/233/examples/advanced/6 mutable. Immutable/security James Tam

Class Security. Example public class Security. Example { private String s; private String. Buffer sb; public Security. Example() { s = new String("Original s"); sb = new String. Buffer("Original sb"); } public String get. S() { return s; } public String. Buffer get. SB() { return sb; } } James Tam

The Driver Class public class Driver { public static void main(String [] args) { Security. Example se = new Security. Example(); String s; String. Buffer sb; System. out. println("Originals"); System. out. println("t" + se. get. S()); System. out. println("t" + se. get. SB()); s = se. get. S(); sb = se. get. SB(); James Tam

The Driver Class (2) sb. delete(0, sb. length()); sb. append("lolz! mucked ur data : P"); s = "lolz! mucked ur data : P"; System. out. println(); System. out. println("After modfications"); System. out. println("Values of locals"); System. out. println("tt. String=" + s); System. out. println("tt. String. Buffer=" + sb); System. out. println("t. Values of attributes"); System. out. println("tt. String=" + se. get. S()); System. out. println("tt. String. Buffer=" + se. get. SB()); } } James Tam

Automatic Garbage Collection Of Java References • Dynamically allocated memory is automatically freed up when it is no longer referenced (Foo = a class) e. g. , Foo f 1 = new Foo(); –Foo f 2 = new Foo(); References Dynamic memory f 1(Address of a “Foo”) Object (Instance of a “Foo”) f 2 (Address of a “Foo”) Object (Instance of a “Foo”) James Tam

Automatic Garbage Collection Of Java References (2) • Dynamically allocated memory is automatically freed up when it is no longer referenced e. g. , f 2 = null; References Dynamic memory f 1 Object (A “Foo”) f 2 Object (A “Foo”) null James Tam

Automatic Garbage Collection Of Java References (3) • Dynamically allocated memory is automatically freed up when it is no longer referenced e. g. , f 2 = null; (recall that a null reference means that the reference refers to nothing, it doesn’t contain an address). References Dynamic memory f 1 Object (A “Foo”) f 2 Object (A “Foo”) null James Tam

Caution: Not All Languages Provide Automatic Garbage Collection! • Some languages do not provide automatic garbage collection (e. g. , C, C++, Pascal). • In this case dynamically allocated memory must be manually freed up by the programmer. • Memory leak: memory that has been dynamically allocated (such as via the Java ‘new’ keyword’) but has not been freed up after it’s no longer needed. –Memory leaks are a sign of poor programming style and can result in significant slowdowns. James Tam

The Finalize() Method • The Java interpreter tracks what memory has been dynamically allocated via ‘new’ • It also tracks when memory is no longer referenced. • When the system isn’t busy, the Automatic Garbage Collector is invoked. • If an object has a finalize method implemented then it is invoked: – The finalize is a method written by the programmer to free up nonmemory resources e. g. , closing and deleting temporary files created by the program, closing network connections. – This method takes no arguments and returns no values (i. e. , returns void) – Dynamic memory is NOT freed up by this method. • After the finalize method finishes execution, the dynamic memory is freed up by the Automatic Garbage Collector. James Tam

The Finalize() Method • Example sequence: public class Foo f 1 { int num; public Foo() { num = 1; } public Foo(int a. Value) { num = a. Value; }. . . Foo f 1 = new Foo (); num 1

The Finalize() Method • Example sequence: public class Foo f 1 { int num; public Foo() { num = 1; } public Foo(int a. Value) { num = a. Value; }. . . Foo f 1 = new Foo(); f 1 = new Foo(10); num 10

The Finalize() Method • Example sequence: public class Foo f 1 { int num; public Foo() { num = 1; } public Foo(int a. Value) { num = a. Value; }. . . Foo f 1 = new Foo(); f 1 = new Foo(10); num 10 Don’t know when

The Finalize() Method • Example sequence: public class Foo f 1 { int num; public Foo() { num = 1; } public Foo(int a. Value) { num = a. Value; }. . . Foo f 1 = new Foo(); f 1 = new Foo(10); f 1. finalize() num 10 Don’t know when

The Finalize() Method • Example sequence: public class Foo f 1 { int num; public Foo() { num = 1; } public Foo(int a. Value) { num = a. Value; }. . . Foo f 1 = new Foo(); f 1 = new Foo(10); f 1. finalize() num 10 Don’t know when

Example Application Of finalize() • As a sheep object gets de-allocated from memory (memory is freed up because the object is no longer referenced) the finalize() method could update the sheep count. public class Sheep { private final int flock. Size = 0; public Sheep() { flock. Size++; } … public void static finalize() { flock. Size--; } }

Displaying The Current State Of Objects • The to. String() method is commonly implemented to allow determination of the state of a particular object (contents of important attributes). • This method returns a string representation of the state of an object. • It will automatically be called whenever a reference to an object is passed as a parameter is passed to the “print()/println()” method.

to. String() Example • Location of the full example: – /home/233/examples/advanced/7 to. String

Class Person public class Person { private int height; private int weight; private String name; public Person(String name, int height, int weight) { this. name = name; this. height = height; this. weight = weight; }

Class Person (2) public String get. Name() { return(name); } public int get. Height() { return(height); } public int get. Weight() { return(weight); }

Class Person (3) public String to. String() { String s; s = "Name: " + name + "t"; s = s + "Height: " + height + "t"; s = s + "Weight: " + weight + "t"; return(s); } }

The Driver Class public class Driver { public static void main(String [] args) { Person jim = new Person("Jim", 69, 160); System. out. println("Atrributes via accessors()"); System. out. println("t" + jim. get. Name() + " " + jim. get. Height() + " " + jim. get. Weight()); System. out. println("Atrributes via to. String()"); System. out. println(jim); } }

Comparing Objects • Recall from the discussion of parameter passing (pass by reference) that a reference contains the address of an object or array. • Using the comparison operator on the references ‘==‘ will only determine if the address (and not data) is the same. String s 1 = “hi”; String s 2 = “hi”; if (s 1 == s 2) String object s 1 “hi” String object s 2 “hi”

Comparing Objects (2) • Either each attribute of each object must be manually compared or else some form of equals() method must be implemented. • Class String has two methods: – compare. To() – compare. To. Ignore. Case() # ABC not same as Abc # ABC same as abc

Implementing Equals() • Location of the full example: – /home/233/examples/advanced/8 equals

Class Person public class Person { private int height; private int weight; public Person(int height, int weight) { this. height = height; this. weight = weight; } public int get. Height() { return(height); } public int get. Weight() { return(weight); }

Class Person (2) public void set. Height(int height) { this. height = height; } public void set. Weight(int weight) { this. weight = weight; } Implicit: Jim Explicit: Bob public boolean equals(Person compare. To) { boolean flag = true; if (this. height != compare. To. get. Height() || this. weight != compare. To. get. Weight()) flag = false; return(flag); } }

The Driver Class public class Driver { public static void main(String [] args) { Person jim = new Person(69, 160); Person bob = new Person(72, 175);

new Person(69, 160); The Driver Class (2) new Person(72, 175); System. out. println("Different data, addresses"); System. out. println("Compare data via accessors()"); if (jim. get. Height() == bob. get. Height() && jim. get. Weight() == bob. get. Weight()) System. out. println("t. Objects same data"); else System. out. println("t. Not equal"); System. out. println("Compare data via equals()"); if (jim. equals(bob) == true) System. out. println("t. Objects same data"); else System. out. println("t. Not equal"); System. out. println("Compare addresses"); if (jim == bob) System. out. println("t. Same address"); else System. out. println("t. Different addresses");

Person(72, 175); # via set() Person(72, 175); The Driver Class (3) System. out. println(); System. out. println("Same data, different addresses"); jim. set. Height(72); jim. set. Weight(175); if (jim. equals(bob) == true) System. out. println("t. Objects same data"); else System. out. println("t. Not equal"); System. out. println("Compare addresses"); if (jim == bob) System. out. println("t. Same address"); else System. out. println("t. Different addresses");

Person(72, 175); # via set() Person(72, 175); The Driver Class (4) System. out. println(); System. out. println("Same data, different addresses"); jim. set. Height(72); jim. set. Weight(175); if (jim. equals(bob) == true) System. out. println("t. Objects same data"); else System. out. println("t. Not equal"); System. out. println("Compare addresses"); if (jim == bob) System. out. println("t. Same address"); else System. out. println("t. Different addresses");

jim = bob; The Driver Class (5) System. out. println(); System. out. println("Same addresses"); jim = bob; if (jim == bob) System. out. println("t. Same address"); else System. out. println("t. Different addresses");

After This Section You Should Now Know • References – How references and objects are related – The difference between a deep vs. shallow copy – How to check for if objects are identical (on a field-by-field basis and by implementing an equals() method – What is the difference between comparing references vs. objects • How the two methods of parameter passing work, what types are passed using each mechanism • What are the benefits of employing the indirect mechanism of references-data vs. just data variables • What is a wrapper class and what is its purpose James Tam

After This Section You Should Now Know (2) • How to pass arrays as parameters and return them from methods • Arrays of 'objects‘ – Why they are really arrays of references – How to declare such an array, create and access elements • How could a simple simulation be implemented using an array of references • How to declare class constants • Static attributes and methods – – How to create statics How to access statics When something should be static vs. non-static (instance) The difference between static and final James Tam

After This Section You Should Now Know (3) • Attributes vs. local variables – How is each one declared – What is the scope of each – How is each one accessed • What are the scoping rules for locals and attributes • What is shadowing • Design issues – When should something be declared as local vs. an attribute – How to determine which attributes and methods should be part of which classes • What is the 'this' reference – When it is and is not an implicit parameter – What's the difference between implicit and explicit parameters – What are the benefits of having a this parameter James Tam

After This Section You Should Now Know (4) • Mutable vs. immutable types – What is the difference – What is the advantage of each type – What is automatic garbage collection • The finalize() method – – How to define one When is it called What are common uses for this method How is it related to automatic garbage collection • How to display the current state of an object by implementing a to. String() method James Tam